1. Field of the Invention
This invention relates generally to forming silicon-carbide tooling and relates specifically to forming tooling used for layup and curing of composite structures.
2. Description of the Prior Art
Tooling for production of composite aircraft parts have close tolerances for dimensional control and are typically made from invar alloy. Invar can be characterized as an expensive material which is difficult to machine. However, an attractive feature of invar is a very low coefficient of thermal expansion (CTE) of approximately 1.5 xcexcin./in./xc2x0 F. at temperatures up to 400xc2x0 F. For applications such as making flat laminates, other tooling materials are sometimes used, including aluminum and steel, and have CTE values of approximately 14 xcexcin./in./xc2x0 F. and 7 xcexcin./in./xc2x0 F., respectively.
A low CTE is necessary for producing high-temperature-cure polymer-matrix/carbon-fiber composites with precise dimensional accuracy. A mismatch in the CTE of the composite material and the tooling material will cause complications for maintaining dimensional accuracy. The CTE of a typical polymer-matrix/carbon-fiber composite is difficult to characterize precisely because it is a multi-component system. Carbon fiber has a small, negative CTE and a typical polymer matrix systems for structural aircraft composite components have a CTE in the range of 15-50 xcexcin./in./xc2x0 F. The CTE of a specific composite system will be dependent upon the lay-up construction and the composition.
Recently, technology has been developed at NASA Glenn Research Center to produce very economical, complex-shaped, silicon carbide (SiC) ceramic structures from wood precursors, called xe2x80x9cecoceramics.xe2x80x9d To produce the ceramics, a wood preform is pyrolyzed in an inert atmosphere to convert the organic material into a carbonaceous form. The preform is infused with liquid silicon or silicon alloy, and the infiltrated preform is converted to a SiC in a high temperatures furnace. These materials have tailorable microstructures, low manufacturing costs, and can be easily machined in precursor stage before forming near-net shape ceramic structures. The targeted applications for this material have been seals, rings and filters for automotive applications; and armor, bricks, foundry crucibles and furnace components. These applications make use of the high service temperature of SiC (1350xc2x0 C.).
One limitation of the NASA process, however, is that it is limited to small pieces of wood, especially those having a thickness of less than 1 in. With larger pieces of wood, cracking and warping are caused as the wood is dried and pyrolyzed, causing loss of tolerance and defects in the structure.
It is generally accepted that a material having a low CTE is desirable for use as composite tooling, and the CTE of SiC is approximately 2.5 xcexcin./in./xc2x0 F. in the temperature range of 70-2282xc2x0 F. There exists a need for low-cost, easily-manufactured, SiC tooling for use in forming composite components of many sizes and thicknesses.
A method is provided for forming items from ecoceramic-based silicon-carbide. A wood preform is machined to a general shape having over- or undersized dimensions. The preform is pyrolyzed to transform the wood of the preform to a porous, carbonaceous material that retains the general shape of the preform. The preform is then machined to final, net-shape dimensions and immersed in liquid silicon or silicon alloy that penetrates and infuses the preform. The infused preform is held at a temperature sufficient to cause the transformation of the material in the preform to silicon carbide, completing formation of the item. Also provided is a method of forming ecoceramic-based tooling and composite components using the tooling.